Hybrid automatic repeat request (HARQ) is widely supported in current state-of-the-art wireless communication standards. Under automatic repeat request (ARQ), error detection information is added to data before transmission, ensuring that the receiver is able to decode the data. With HARQ, additional forward error correction (FEC) bits are also added to the data. From the timing relationship point of view, HARQ can be categorized as asynchronous HARQ and synchronous HARQ.
Asynchronous HARQ refers to the case that (re)transmissions for a certain HARQ process may occur at any time. Explicit signaling of the HARQ process number is therefore required for asynchronous HARQ.
Synchronous HARQ refers to the case where (re)transmissions for a certain HARQ process are restricted to occur at known time instants. No explicit signaling of the HARQ process number is required for synchronous HARQ, because the process number may be derived from, for example, the sub-frame number.
In addition to the categorization of HARQ as either synchronous or asynchronous, HARQ timing is also defined to allow precise operation of the transmitter and receiver. The timing relationship includes the relationship between control signaling, associated transmission, associated feedback, and associated retransmission. Such timing relationships may be defined explicitly or implicitly. For explicit timing relationships, the control signaling includes some fields to indicate the timing relationship. Implicit timing relationships are implemented by defining some rules on timing.
Several wireless communication standards are defined by the Institute of Electrical and Electronics Engineers (IEEE), including 802.16e (broadband wireless access) and 802.16m (advanced air interface standard). Under IEEE 802.16e, asynchronous HARQ is used for both downlink (DL) and uplink (UL) transmissions. Under 802.16m, asynchronous HARQ is used in the downlink while synchronous HARQ is used for uplink transmissions. There are various factors affecting HARQ timing:                Number of sub-frames per frame: in 5/10/20 MHz systems bandwidth, there are 8 sub-frames per frame. However, in 7 and 8.75 MHz system bandwidth, there are only 6 and 7 sub-frames per frame, respectively        Variable transmission time interval (TTI) length: one TTI might contain multiple sub-frames        Legacy IEEE 802.16e zones: when mixed with legacy zones, the available downlink/uplink sub-frames for 802.16m transmissions are different        Relay zones: HARQ timing should be defined for the base station (BS), the mobile station (MS), and the relay station (RS)        Availability of an acknowledge channel (ACKCH): when n=2 is configured for a user-specific control channel (USCCH), some downlink sub-frames do not contain USCCH, such that the downlink ACKCH cannot be transmitted (USCCH is also known as assignment A-MAP)        